Here's why it's difficult to say:
* His life was long ago: He lived from 1774 to 1845, a time when American English was evolving rapidly.
* Regional variation: His travels across the country would have exposed him to many regional accents.
* Lack of recordings: There are no audio recordings of him speaking, so we have to rely on written accounts and descriptions.
However, it's safe to assume his accent would have been:
* Rural and Midwestern: He spent most of his life in what is now Ohio, Indiana, and Illinois, areas known for their distinctive accents.
* Possibly influenced by the Scots-Irish: His family was of Scots-Irish descent, which could have influenced his speech.
Without a time machine, we'll never know exactly what Johnny Appleseed sounded like, but we can imagine a friendly, down-to-earth voice with a touch of Midwestern twang.